This is a U.S. national stage of application No. PCT/EP00/11617, filed on Nov. 22, 2000. Priority is claimed on that application and on the following applications: Country: Germany, Applications No.: 199 59 141.5, filed: Dec. 08, 1999 and Country: Germany, Application No. 100 47 815.8, filed Sep. 27, 2000.
The invention concerns a device with multiple exhaust gas supply lines for supplying exhaust gases from an internal combustion engine to a catalytic converter, especially a storage catalytic converter, in accordance with the introductory clause of claim 1.
To convert nitrogen oxides contained in automotive exhaust gases from Otto engines with direct fuel injection, NOx storage catalytic converters are preferably used. However, these provide sufficiently high conversion rates only within a relatively narrow window of exhaust gas temperatures. This results in a disadvantageous limitation of the high-performance lean operating range of an Otto engine. To achieve optimum reduction of the fuel consumption of direct-injection Otto engines by lean operation, the exhaust system must be adjusted to the engine in such a way that the exhaust gas temperatures immediately before the inlet to the storage catalytic converter fall within a well-defined window of operating temperatures over the greatest possible engine operating range. The upper limit of this window of operating temperatures is reached at high vehicle speeds or high engine loads. Exceeding this upper temperature limit has a negative impact on the operation and service life (durability) of the storage catalytic converter. Therefore, to extend the lean operating range of the Otto engine, the exhaust gases must be cooled before they enter the storage catalytic converter at high vehicle speeds or high engine loads. On the other hand, it is also necessary to ensure that, after a cold start of the direct-injection Otto engine, the storage catalytic converter reaches its so-called catalytic ignition point as fast as possible, i.e., the operating temperature for optimum pollutant conversion. To this end, it is necessary to take suitable measures, which, immediately after a cold start of the Otto engine, allow the least possible removal of heat from the exhaust gases to the surrounding environment before the exhaust gases enter the catalytic converter.
DE 199 05 345 A1 describes a generic device for supplying exhaust gases from an internal combustion engine to an exhaust gas treatment system with multiple exhaust gas supply lines. Several exhaust gas supply lines are installed between a three-way catalytic converter and an NOx trap. These multiple supply lines are designed to retain a relatively large proportion of the heat in the stream of exhaust gas at relatively low exhaust gas mass flows, whereas at higher mass flows, a relatively greater proportion of the heat is eliminated with the stream of exhaust gas.
Therefore, the goal of the present invention is to specify a generic device that allows reliable adjustment of the exhaust gas temperatures at the inlet of the exhaust gases into the catalytic converter.
This goal is achieved by a device with the features specified in claim 1. This device is characterized by the fact that at least one exhaust gas supply line is designed as a heat-conveying line that promotes transfer of the exhaust gas heat to the catalytic converter, that at least one other exhaust gas supply line is designed as a heat exchanger that promotes extraction of heat from the exhaust gas upstream of the catalytic converter, and that a switchable control mechanism is provided to control a flow cross section of at least one of the exhaust gas supply lines. A device designed in this way is advantageously suited for maintaining the exhaust gas temperatures within a well-defined window of operating temperatures at the inlet of the catalytic converter for the greatest possible engine operating range (high vehicle speeds or high engine loads) as well as immediately after a cold start of the engine. This is accomplished with the use of a switchable control mechanism for controlling a flow cross section of at least one of the exhaust gas supply lines, i.e., the heat-conveying line or the heat exchanger. In this connection, the purpose of the heat-conveying line is to bring the catalyst to the catalytic ignition point as quickly as possible after a cold start of the Otto engine, while the purpose of the heat exchanger is to cool the exhaust gases upstream of the catalytic converter and thus to prevent unwanted exceeding of the upper temperature limit of the operating temperature window, especilly at high engine loads. During a cold start of the Otto engine, it is possible, for example, for the heat-conveying line to be open (maximum flow cross section) to allow maximum transfer of the heat of the exhaust gases to the catalytic converter, and for the heat exchanger to be partially or completely closed, so that as little exhaust gas heat as possible is transferred to the surrounding environment upstream of the catalytic converter and is used instead to heat the catalyst and thus to bring it to its ignition point as soon as possible after the cold start of the Otto engine. When the catalyst has reached the catalytic ignition point, and the Otto engine is being subjected to a higher load (higher vehicle speeds), it is advantageous to open the heat exchanger partially or completely and possibly to close the heat-conveying line partially or completely to prevent the upper temperature limit of the window of operating temperatures from being exceeded due to the transfer of too much heat to the catalytic converter by the exhaust gases. All of these adjustments with respect to the flow cross section of one or more exhaust gas supply lines are adapted to the given engine operating range for the purpose of controlling or regulating the amount of heat transferred to the catalytic converter by the exhaust gases and can be accomplished easily, precisely, and reliably by a switchable control mechanism.
The exhaust gas supply lines are preferably configured as pipes that are separated from one another. Since the exhaust gas supply lines are designed to have two different or opposing purposes, i.e., optimum transfer of exhaust gas heat to the catalytic converter (heat-conveying line) and minimum transfer of exhaust gas heat to the catalytic converter (heat exchanger), depending on the given engine operating range (cold start, higher engine load), it is advantageous to design the exhaust gas supply lines as separate pipes, which, preferably, are also spaced slightly apart. Separation of the heat-conveying line from the heat exchanger makes it possible to realize especially effective adjustment of the heat transfer to the catalytic converter by the exhaust gases.
In accordance with a refinement of the invention, the exhaust gas supply line that is designed as a heat-conveying line has a smaller flow cross section than the exhaust gas supply line that is designed as a heat exchanger. In this way, the surface area of heat transfer from the exhaust gas to the surrounding environment upstream of the catalytic converter can be limited to the smallest possible value with respect to the heat-conveying line and can be designed to be as large as possible with respect to the heat exchanger.
The exhaust gas supply lines are advantageously arranged as a bundle of pipes, the individual pipes of which are spaced some distance apart. The supply line that is designed as a heat-conveying line is arranged as an inner pipe in the bundle and is surrounded around its circumference by several exhaust gas supply lines that are designed as heat exchangers. Exhaust gas supply lines that are spaced some distance apart and grouped as a bundle of pipes allow a compact design of the device. In this type of arrangement, the exhaust gas supply lines that are designed as heat exchangers are preferably arranged on the outside of the bundle around the circumference of the inner heat-conveying line or lines to allow the most effective possible heat exchange between the heat exchanger and the surrounding environment.
It is advantageous for the exhaust gas supply line that is designed as the heat-conveying line to be provided with thermal insulation. The use of thermal insulation for the heat-conveying line is an especially simple and effective means of minimizing the loss of exhaust gas heat to the surrounding environment through the heat-conveying line. This ensures the best possible transfer of exhaust gas heat to the catalytic converter. This is especially advantageous during a cold start of the Otto engine, so that the catalyst heats up to the catalytic ignition point as fast as possible.
In accordance with a preferred embodiment of the invention, the exhaust gas supply line that is designed as the heat-conveying line extends into the catalytic converter to a point just upstream of the catalytic support body. This measure makes it possible to achieve rapid heating of a locally limited region of the catalytic support material, so that the temperature of the catalyst rises as fast as possible to the catalytic ignition point, at which the desired reaction of the pollutants occurs (pollutant conversion).
In a modification of the invention, the wall of the exhaust gas supply line that is designed as a heat exchanger is configured in such a way that the heat-transfer surface is increased and/or turbulence is produced in the stream of exhaust gas in the exhaust gas supply line. This serves to intensify the cooling effect of the exhaust gas supply line that is designed as a heat exchanger with respect to the gases to be supplied to the cataltyic converter. The production of exhaust gas turbulence in the heat exchanger prevents the undesirable formation of thermal stratification in the given exhaust gas supply line due to the development of a cold outer flow and a hot inner flow of the exhaust gases. It becomes more and more important to counteract this sort of thermal stratification with increasing length of the exhaust gas supply line in question (heat exchanger).
It is advantageous to provide the exhaust gas supply line that is designed as a heat exchanger with a coating that promotes heat dissipation. A coating of this type is a relatively simple means of achieving an effective increase in heat loss to the surrounding environment, especially radiational heat loss.
In accordance with a preferred embodiment of the invention, the outer surface of the exhaust gas supply line that is designed as a heat exchanger is acted on by an outside airflow that promotes heat loss. This results in an additional increase in the radiational heat loss by the heat exchanger to the surrounding environment. In this regard, a suitable configuration of the external wall of the exhaust gas supply line that is designed as a heat exchanger can produce desirable turbulence of the outside air along the wall of the pipe to produce especially effective dissipation of heat by the heat exchanger to the surrounding environment. An outside airflow of this sort can be produced, for example, by systematic guidance of the airstream through an air intake in the front end of the vehicle or the subfloor cover of the vehicle.
In accordance with a preferred embodiment of the invention, the control mechanism has a switching element that can be controlled by a control element as a function of various parameters. Parameters that may bemused to control a flow cross section of at least one of the exhaust gas supply lines are the engine coolant temperature, the exhaust gas temperature upsteam and/or downstream of the catalytic converter (with respect to the direction of exhaust gas flow), the catalytic converter temperature, and/or the exhaust gas mass flow. The purpose of using the exhaust gas mass flow as a control parameter is to avoid the development of unacceptably high exhaust gas back pressures at excessively high exhaust gas mass flows.
The switching element is advantageously designed as a rotatable disk with holes corresponding to the pipes in the bundle of pipes. A rotatable disk with holes corresponding to the supply lines is well suited for simultaneous and precise control, by proper rotation of the disk, of each flow cross section of preferably several exhaust gas supply lines as a function of various parameters. Designing the switching element in the form of a disk makes it possible to realize an especially compact device.
In accordance with a preferred embodiment of the invention, the control element is designed as a control rod actuated by an electric servomotor. A control element designed in this way allows reliable and precise control (rotation) of the switching element.
Control (rotation) of the switching element designed as a disk can be advantageously accomplished by means of a control element adapted to the specific design conditions that are present. In this connection, the control element may be designed, for example, as a vacuum box to produce precise and reliable rotation of the switching element.
In accordance with another, alternative embodiment of the invention, the control element is designed as an electromagnetic control device, which has a winding, by means of which the switching element, which acts as the armature, can be caused to rotate. A control mechanism that operates in this way has an especially compact construction and at the same time allows precise and reliable control of the flow cross section of one or more exhaust gas supply lines.
In addition, the control element may also be hydraulic in design.
The switching element can be advantageously controlled by two-position switching. With a two-position switch, the appropriate exhaust gas supply lines are completely closed or completely open, depending on present need (cold start, higher engine load), such that, if necessary, it is possible to differentiate between the exhaust gas supply line or lines that are designed as heat-conveying lines and as heat exchangers. Two-position switching is a relatively easily realized switching system for controlling the switching element.
In accordance with an alternative embodiment of the invention, the switching element can be adjusted to infinitely many intermediate positions. Continuous control of the flow cross section of the appropriate exhaust gas supply line or lines allows extremely fine adjustment of the temperature of the catalytic converter, especially the NOx storage catalytic converter, to an optimum operating temperature over a relatively broad engine operating range. Here again, it is possible to differentiate between the exhaust gas supply lines that have the function of a heat-conveying line or of a heat exchanger.
The exhaust gas supply line that is designed as a heat-conducting line is advantageously arranged coaxially with respect to the axis of rotation of the switching element. In accordance with this embodiment of the invention, the device contains a heat-conveying line that is arranged coaxially with respect to the axis of rotation of the switching element and with respect to a corresponding hole in the switching element. Rotation of the switching element thus causes no adjustment of the flow cross section of the heat-conveying line, but rather only of the flow cross sections of the outer exhaust gas supply lines that are designed as heat exchangers. Consequently, in this embodiment of the invention, the heat-conveying line is always in an open state, whereas the exhaust gas supply line or lines that are designed as heat exchangers may be opened, closed, or possibly partially closed (with respect to the supply of exhaust gases to the catalytic converter) by adjustment of the switching element.
In accordance with an alternative embodiment of the invention, the exhuast gas supply line that is designed as a heat-conveying line is positioned a certain radial distance from the axis of rotation of the switching element. In this embodiment of the invention, the flow cross sections of both the heat-conveying line and the line designed as a heat exchanger, which is positioned towards the outside, are changed, because all of the holes in the switching element are spaced a certain radial distance from the axis of rotation. Therefore, rotation of the switching element causes relative movement of all of the holes with respect to the corresponding openings of the exhaust gas supply lines (heat-conveying line and heat exchanger).
The switching element advantageously has a central disk and an outer ring that is coaxial with the disk and can be rotated relative to the disk. The disk is provided with holes corresponding to the exhaust gas supply lines that are designed as heat-conveying lines, and the ring is provided with holes corresponding to the exhaust gas supply lines that are designed as heat exchangers. In this way, it is possible for the disk and the ring to be controlled independently of each other to control the corresponding flow cross sections of the corresponding exhaust gas supply lines (heat-conveying line, heat exchanger), so that the desired operating temperature of the catalytic converter can be adjusted with great precision.
The switching element designed as a disk is preferably located upstream of the catalytic converter directly in front of its inlet. This makes it possible to achieve a constructionally relatively simple design of the device, since at least the exhaust gas supply line or lines that are designed as heat exchangers are in active contact with the corresponding holes in the switching element on only one side (inlet side with respect to the switching element).